1. Field of the Invention
The present invention relates to an abnormality detecting apparatus for wheel speed sensors and an automobile incorporating the same. More specifically, it relates to an abnormality detecting apparatus for wheel speed sensors with detection performance of high accuracy and an automobile incorporating the same.
2. Description of the Background Art
Conventionally, wheel speed sensors for detecting the wheel speed of each wheel are incorporated in an ABS (Antilock Brake System) apparatus that prevents wheels from locking during braking, or in an automatic braking apparatus that generates braking force for preventing slip or generating yaw moment to stabilize the attitude of the vehicle that is becoming unstable. As normal control of those apparatuses is lost if there is abnormality with the wheel speed sensors, abnormality detection of the wheel speed sensors is frequently conducted.
As means for detecting abnormality of wheel speed sensors, for example, Japanese Patent Laying-Open No. 11-291886 discloses an abnormality detecting apparatus for wheel speed sensors, of which object is to improve detection accuracy.
FIG. 4 is a control block diagram of the abnormality detecting apparatus for wheel speed sensors described in Japanese Patent Laying-Open No. 11-291886.
Referring to FIG. 4, the abnormality detecting apparatus for wheel speed sensors includes wheel speed sensors 70a outputting signals respectively responsive to rotational speeds of wheels FL, FR, RL, and RR (not shown), wheel speed calculating means 70b for calculating wheel speeds based on the signals from respective wheel speed sensors 70a, wheel acceleration calculating means 70c for calculating wheel acceleration, wheel speed sensor abnormality determining means 70d for determining abnormality of wheel speed sensors 70a based on a result of the calculation by wheel speed calculating means 70b, and wheelspin determining means 70e for determining wheelspin of driving wheels based on a result of the calculation by wheel accelerations calculating means.
In this configuration, wheel speed sensor abnormality determining means 70d involves, as to the result of calculation of the wheel speeds, an abnormality determination condition as follows: while the rotational speed of at least one wheel is at least a prescribed speed, the rotational speed of other wheel is 0 km/h. Specifically, wheel speed sensor abnormality determining means 70d obtains an assumed vehicle speed based on signals from respective wheel speed sensors 70a, and compares the assumed vehicle speed and detected wheel speeds detected by respective wheel speed sensors 70a. In response to a difference between them exceeding a prescribed value being continued for at least a prescribed period, wheel speed sensor abnormality determining means 70d determines that wheel speed sensors 70a are abnormal. Here, wheel speed sensor abnormality determining means 70d determines that a wheel speed sensor 70a corresponding to a wheel having a rotational speed of 0 km/h is abnormal.
However, on a road surface with a low friction coefficient (hereinafter also referred to as low μ) such as of a snow-covered road, if a state where a vehicle hardly moves while the driving wheels are slipping (the wheelspin state) is continued, wheel speed sensors 70a of the driving wheels show detection values of high speeds, whereas wheel speed sensors 70a of idler wheel show detection values of substantially 0 km/h. When this wheelspin state is continued for a prescribed period, wheel speed sensor abnormality determining means 70d determines that wheel speed sensors 70a, which are actually normal, are abnormal.
Accordingly, the abnormality detecting apparatus for wheel speed sensors shown in FIG. 4 is configured such that, when wheelspin determining means 70e determines wheelspin, a determination operation of wheel speed sensor abnormality determining means 70d is suspended. Specifically, wheelspin determining means 70e determines that wheelspin has occurred when wheel acceleration of the driving wheels has exceeded the wheel acceleration that would be possible during normal road-gripping driving. Wheelspin determining means 70e has a wheelspin timer (not shown) for measuring a predetermined time, and while the wheelspin timer is measuring the predetermined time, an abnormality determination operation by wheel speed sensor abnormality determining means 70d is suspended.
With such a configuration, the abnormality detecting apparatus for wheel speed sensors can prevent erroneous abnormality determination due to wheelspin of the driving wheels.
As above, the conventional abnormality detecting apparatus for wheel speed sensors is characterized in that a result of the determination by wheelspin determining means 70e is considered as a condition for establishing an abnormality determination operation at wheel speed sensor abnormality determining means 70d in order to improve the detection accuracy.
However, the conventional abnormality detecting apparatus for wheel speed sensors involves the following problem as to the determination accuracy of wheelspin determining means 70e itself.
In general, wheel acceleration can be obtained from the following equation expressing driving force exerted on the driving wheels:I·dω=T−μ·W·R  (1)
where, I: wheel inertia (kg/m2), do: wheel acceleration (m/s2), T: input torque (N·m), μ: road surface frictional coefficient, W: weight on the ground (N), R: wheel radius (m).
As apparent from equation (1), wheel acceleration dω largely changes depending on input torque T (proportional to acceleration pedal depression), road surface frictional coefficient μ, and weight on the ground W. On the other hand, actually it is difficult to accurately detect road surface frictional coefficient μ and weight on the ground W, and it is not easy to accurately determine wheel acceleration dω. Therefore, in the conventional abnormality detecting apparatus for wheel speed sensors, wheelspin determining means 70e that is based on wheel acceleration dω yields varied determination result, and the abnormality detection accuracy is not always high.
Meanwhile, among four-wheel-drive (4WD) vehicles, what is called standby 4WD is known, which normally travels employing either front wheels or rear wheels as driving wheels and, when relative rotation occurs between the front and rear wheels due to slip or the like, motive power is transmitted to the other rear or front wheels to achieve four-wheel driving.
Specifically, with standby 4WD, rotational speeds of the driving wheels and rotational speeds of the idler wheels are detected, and based on the difference between them, a relative rotation number between the driving wheels and the idler wheels is detected. Further, transmission torque (differential limiting torque) in accordance with the detected relative rotation number is transmitted to the idler wheels via a power transmission, whereby the idler wheels are driven.
In such a configuration, the rotational speeds of the driving wheels and that of the idler wheels are respectively detected by wheel speed sensors arranged at respective wheels. Accordingly, with standby 4WD, it is critical that the wheel speed sensors are normal in order to correctly control switching between two-wheel drive (2WD) and four-wheel drive. When the wheel speed sensors are abnormal, actuation of four-wheel drive is prohibited, since correct control cannot be continued. Specifically, with standby 4WD, based on abnormality detection information on the wheel speed sensors, prohibition/permission of actuation of four-wheel drive is determined. Here, if the abnormality detection accuracy of the abnormality detecting apparatus for wheel speed sensors is low, the wheel speed sensors that are actually normal are erroneously determined to be abnormal, whereby actuation of four-wheel drive is prohibited and the drivability may be decreased. Accordingly, particularly high detection accuracy is required for the abnormality detecting apparatus for wheel speed sensors incorporated in a vehicle employing standby 4WD scheme.
However, as described above, the conventional abnormality detecting apparatus for wheel speed sensors cannot yet guarantee full detection accuracy, as the wheelspin determination is difficult.